The Mackinac Bridge (/ˈmækɨnɔː/ MAK-in-aw) is a suspension bridge spanning the Straits of Mackinac to connect the Upper and Lower peninsulas of the U.S. state of Michigan. Opened in 1957, the 8,614-foot (2,626 m) bridge (familiarly known as "Big Mac" and "Mighty Mac") is the world's fifth-longest in total suspension^[3] and the longest suspension bridge between anchorages in the Western hemisphere.^[3] The Mackinac Bridge carries Interstate 75 and the Lakes Michigan and Huron components of the Great Lakes Circle Tours across the straits and connects the city of St. Ignace on the north end with the village of Mackinaw City on the south.

Envisioned since the 1880s, the bridge was designed by the engineer David B. Steinman and completed in 1957 only after many decades of struggles to begin construction.

Length

The bridge opened on November 1, 1957, connecting two peninsulas linked for decades by ferries. A year later, the bridge was formally dedicated as the "world's longest suspension bridge between anchorages", allowing a superlative comparison to the Golden Gate Bridge, which had a longer center span between towers, and the San Francisco–Oakland Bay Bridge, which had an anchorage in the middle.

It remains the longest suspension bridge with two towers between anchorages in the Western Hemisphere.^[3] Much longer anchorage-to-anchorage spans have been built in the Eastern Hemisphere, including the Akashi Kaikyō Bridge in Japan (12,826 ft or 3,909 m). But the long leadups to the anchorages on the Mackinac make its total shoreline-to-shoreline length of 5 miles (8 km) longer than the Akashi-Kaikyo (2.4 mi or 3.9 km).

The length of the bridge's main span is 3,800 feet (1,158 m), which makes it the third-longest suspension span in the United States and 15th longest worldwide. (See also List of longest bridges in the world.)

History

The Algonquian peoples Native Americans who lived in the straits area prior to the arrival of Europeans in the 17th century called this region Michilimackinac, which is widely understood to mean the Great Turtle presumably named for the shape of Mackinac Island. This interpretation of the word is debated by scholars. The Straits of Mackinac functioned as a fur trade summer depot and inter-tribal meeting place.^[4]

As exploitation of the state's mineral and timber resources increased during the 19th century, the area became an important transport hub. In 1881 the three railroads that reached the Straits, the Michigan Central, Grand Rapids & Indiana, and the Detroit, Mackinac & Marquette, jointly established the Mackinac Transportation Company to operate a railroad car ferry service across the straits.^[5]

Improved highways along the eastern shores of the lower peninsula brought increased automobile traffic to the straits region starting in the 1910s. The state of Michigan initiated an automobile ferry service between St. Ignace and Mackinaw City in 1923 and eventually operated nine ferry boats that would carry as many as 9,000 vehicles per day, with traffic backups stretching as long as 16 miles (26 km).^[6]

After the opening of the Brooklyn Bridge in 1883, local residents began to imagine that such a structure could span the straits. In 1884, a store owner in St. Ignace published a newspaper advertisement that included a reprint of an artist's conception of the Brooklyn Bridge with the caption "Proposed bridge across the Straits of Mackinac".^[7]

At a July 1888 meeting of the board of directors of the Grand Hotel on Mackinac Island, Cornelius Vanderbilt II proposed the building of a bridge, similar to the one then under construction across the Firth of Forth in Scotland, across the straits in order to advance commerce and to help lengthen the resort season of the hotel.^[7]

The idea of the bridge was discussed in the Michigan Legislature as early as the 1880s. At the time, the area was becoming a popular tourist destination, especially following the creation of Mackinac National Park on Mackinac Island in 1875.

Despite the perceived necessity for the bridge, decades elapsed with no formal plan. In 1920, the Michigan state highway commissioner advocated the construction of a floating tunnel across the straits. At the invitation of the state legislature, C. E. Fowler of New York City put forth a plan for a long series of causeways and bridges across the straits from Cheboygan, 17 miles (27 km) southeast of Mackinaw City, to St. Ignace, using Bois Blanc, Round, and Mackinac Islands as intermediate steps.

In 1923, the state legislature ordered the State Highway Department to establish ferry service across the strait. More and more people used ferries to cross the straits each year, and as they did, momentum to create a bridge grew even stronger. Chase Osborn, a former governor, wrote, "Michigan is unifying itself, and a magnificent new route through Michigan to Lake Superior and the Northwest United States is developing, via the Straits of Mackinac. It cannot continue to grow as it ought with clumsy and inadequate ferries for any portion of the year."

By 1928, the ferry service had become so popular and so expensive to operate that Michigan Governor Fred Green ordered the department to study the feasibility of building a bridge across the strait. The department deemed the idea feasible, estimating the cost at $30 million.

In 1934, the Michigan Legislature created the Mackinac Straits Bridge Authority to explore possible methods of constructing and funding the proposed bridge. The Legislature authorized the Authority to seek financing for the project. In the mid-1930s, the Authority twice attempted to obtain federal funds for the project but was unsuccessful, despite the endorsement of the United States Army Corps of Engineers and President Franklin D. Roosevelt. Nevertheless, between 1936 and 1940, a route was selected for the bridge, and borings were made for a detailed geological study of the route.

Besides the length of the span, a unique engineering challenge is the tremendous forces that operate against the base of the bridge. During winter, the lakes freeze and enormous stress is placed on the bridge structure by large icebergs.

The preliminary plans for the bridge featured a 3-lane roadway, a railroad crossing on the underdeck of the span, and a center-anchorage double-suspension bridge configuration similar to the design of the San Francisco – Oakland Bay Bridge. Because this would have required sinking an anchorage pier in the deepest area of the Straits, the practicality of this design may have been questionable.^{[citation needed]} A concrete causeway, approximately 4,000 feet (1,219 m), extending from the northern shore, was constructed in shallow water from 1939 to 1941. At that time, with funding for the project still uncertain, further work was put on hold because of World War II. The Mackinac Straits Bridge Authority was abolished by the state legislature in 1947, but the same body created a new Mackinac Bridge Authority three years later in 1950. In June 1950, engineers were retained for the project. After a report by the engineers in January 1951,^[8] the state legislature authorized the sale of $85 million in bonds for bridge construction on April 30, 1952. However, a weak bond market in 1953 forced a delay of more than a year before the bonds could be issued.

G. Mennen Williams was governor during the construction of the Mackinac Bridge. He began the tradition of the governor leading the Mackinac Bridge Walk across it every Labor Day.^[9] U.S. Senator Prentiss M. Brown has been called the "father of the Mackinac Bridge,"^[10] and was honored with a special memorial bridge token created by the Mackinac Bridge Authority.^[11]

Engineering and construction

David B. Steinman was appointed as the design engineer in January 1953 and by the end of 1953, estimates and contracts had been negotiated. Total cost estimate at that time was $95 million with estimated completion by November 1, 1956. Tolls collected were to pay for the bridge in 20 years.^[12] Construction began on May 7, 1954. The American Bridge Division of United States Steel Corporation was awarded a contract of more than $44 million to build the steel superstructure.

Construction, staged using the 1939-41 causeway, took three and a half years (four summers, no winter construction) at a total cost of $100 million and the lives of five workers. Contrary to popular belief, none of them are entombed in the Bridge.^[13] It opened to traffic on schedule on November 1, 1957, and the ferry service was discontinued on the same day. The Bridge was formally dedicated on June 25, 1958. The bridge officially achieved its 100 millionth crossing exactly forty years after its dedication, on June 25, 1998.^[14]

The 50th anniversary of the bridge's opening was celebrated on November 1, 2007 in a ceremony hosted by the Mackinac Bridge Authority at the viewing park adjacent to the St. Ignace causeway.^[14]

History of the bridge design

The design of the Mackinac Bridge was directly influenced by the lessons from the first Tacoma Narrows Bridge, which failed in 1940 because of its instability in high winds. Three years after that disaster, Steinman had published a theoretical analysis of suspension-bridge stability problems, which recommended that future bridge designs include deep stiffening trusses to support the bridge deck and an open-grid roadway to reduce its wind resistance. Both of these features were incorporated into the Mackinac Bridge. The stiffening truss is open to reduce wind resistance. The road deck is shaped as an airfoil to provide lift in a cross wind, and the center two lanes are open grid to allow vertical (upward) air flow, which fairly precisely cancels the lift, making the roadway stable in design in winds of up to 150 miles per hour (240 km/h).

Sumber :

http://en.wikipedia.org/wiki/Mackinac_Bridge